To enhance the protection and beauty of a display in flat display panel devices such as digital cameras, mobile phones or personal digital assistants (PDAs), a thin plate-shaped cover glass is arranged on the front surface of a display such that a region of the cover glass is wider than an image display part. Glass has high theoretical strength, but the strength is greatly decreased by scratches made thereon. For this reason, a chemically strengthened glass having a compressive stress layer formed on the glass surface by ion exchange or the like is used as a cover glass requiring strength.
With demands of reduction in weight and reduction in thickness to a flat panel display device, a cover glass itself is required to decrease its thickness. Therefore, for the cover glass, further strength is required in main surfaces (front and back surfaces) and an edge surface in order to satisfy the object thereof.
As strength of a glass, for example, surface strength is regarded as an index of strength of glass main surfaces, and for example, bending strength is regarded as an index of strength of glass edge surface.
Regarding surface strength of a glass, it is known that the surface strength of a glass is decreased by the presence of hydrogen (water) in a glass (Non-Patent Documents 1 and 2).